1. Field of the Invention
The present invention relates to an optical clock phase-locked loop circuit that is preferably used during recovery of an optical clock and so forth in an optical transmission system.
2. Background Art
The majority of current optical communication systems employ digital transmission systems. For digital transmission systems, attenuation and waveform distortion of optical signal received from the transmission line is regenerated by arranging regenerators at fixed intervals along the transmission line. As a result, noise and distortion applied to the signal is limited within a single span, thereby making it possible to prevent deterioration of the signal-to-noise ratio caused by accumulation of noise and waveform distortion, and enabling high-quality signals to be sent over long distances.
In order to realize such digital transmission systems, a timing extraction function becomes important that determines the accurate time position of the optical signal. In a transmission system, since timing fluctuations of the optical signal occur due to temperature changes in the transmission line and other changes in the external environment, it is necessary to extract the timing from the optical signal after transmission.
An example of this type of timing extraction technology is shown in FIG. 13 which indicates an example of a optical clock phase-locked loop circuit (optical PLL circuit) of the prior art. In this drawing, reference symbol 1 indicates an optical signal input terminal, 2 an optical coupler, 3 an optical cross-correlation detection device, 4 an optical band pass filter, 5 an optical receiver, 6 a phase comparator, 7 a voltage controlled oscillator (VCO), 20 a microwave mixer, 21 and 24 optical pulse generators, 22 a low-frequency oscillator, 13 an optical clock output terminal, 23 a frequency doubler, and 25 an optical amplifier.
The following provides an explanation of the operation of the optical clock phase-locked loop circuit of the prior art shown in FIG. 13. The frequency of the output signal of VCO 7 is shifted by low-frequency oscillator 22 and microwave mixer 20, and drives optical pulse generator 21 to generate optical clock having a repetition frequency of f0+Δf. The waveform of the optical clock is not required to be a sine wave, but rather is required to have a narrow pulse width and contain a harmonic component n(f0+Δf) (provided n is an integer of 2 or more) of a repetition frequency in its spectrum. The nΔf component is generated by detecting the cross-correlation signal between this nth harmonic n(f0+Δf) and optical signal of a bit rate nf0. A phase comparison is then performed between this nΔf output and nΔf signal for which the output of low-frequency oscillator 22 has been multiplexed n times, and PLL operation is achieved by feeding this back to VCO 7.
The details of this principle are described in Japanese Unexamined Patent Application, First Publication No. 7-287264 entitled, “Optical Cross-Correlation Detection Circuit and Optical Clock Phase-Locked Loop Circuit”.
However, the repetition frequency of the optical clock output from optical pulse generator 21 in the optical PLL circuit is f0+Δf, and is not synchronized with bit rate nf0 of the optical signal. Consequently, in order to obtain optical clock that has been synchronized with the optical signal, a different optical pulse generator 24 must be driven by the output of VCO 7 (repetition frequency f0), and amplified by optical amplifier 25 as necessary. In this case, there is the problem of deterioration of the stability of the optical clock that is output due to jitter of optical pulse generator 24 and temperature changes and so forth of the optic fiber in optical amplifier 25 composed of an optic fiber amplifier and so forth.
In order to solve the problems, the object of the present invention is to provide an optical clock phase-locked loop circuit capable of realizing stable operation as compared with the prior art.